For 117 years the Boston Athletic Association has offered athletes the physical challenge of replicating Pheidippides’s decision to run the distance from Marathon to Athens, on a bloody late-summer day in 490 BC. But next year, for the first time, the athletes who take part in the 118th Boston Marathon will, in effect, be accepting his actual mission, which was to convey a one-word message: “Nikomen,” or “We have won.”